As steel parts and methods for their manufacture are known those described in Patent Documents 1 and 2 listed below. The technology set out in Patent Document 1 is comprised so as to place a steel part in a sealed container charged with hydrocarbon gas, heat all but corner portions, and conduct ambient heat to raise the corner portions to a temperature lower than a surrounding flat portion, thereby inhibiting precipitation of cementite at the corner portions and achieving manufacture of a steel part excellent in toughness.
The technology set out in Patent Document 2 comprises charging mixed gas of hydrogen and nitrogen into a chamber accommodating a work to perform atmosphere replacement, lowering dew point to minus 50° C. or below, and thereafter initiating carburization by supplying a mixed gas of acetylene and nitrogen and thus manufacturing an unevenness-free carburized product incurring no hindrance of carburization by oxide film.